In order to improve the accuracy of an air-fuel ratio control of an internal combustion engine, as shown in Japanese Patent No. 2684011 (U.S. Pat. No. 5,542,404), there has been performed a cylinder air-fuel ratio control that performs the cylinder air-fuel ratio estimate of estimating the air-fuel ratio of each cylinder by the use of a model for relating the detection value of one air-fuel sensor disposed in an exhaust confluent portion where exhaust gases from plural cylinders merge with each other (air-fuel ratio of the exhaust confluent portion) to the air-fuel ratio of each cylinder, and computes an air-fuel ratio correction quantity for each cylinder so as to reduce variation in the air-fuel ratio of each cylinder between the cylinders based on the estimate result of the cylinder air-fuel ratio estimate and controls the air-fuel ratio of each cylinder (fuel injection quantity) based on the air-fuel ratio correction quantity for each cylinder. In a control unit described in Japanese Patent No. 2684011, it is performed the cylinder abnormality diagnosis of determining whether or not the air-fuel ratio feedback correction quantity for each cylinder is within a specified range, and determining that an abnormality (for example, malfunction of an injector) occurs in a cylinder when an air-fuel ratio feedback correction quantity for the cylinder is beyond the specified range.
A hit condition of the exhaust gas as opposed to the air fuel ratio sensor may be changed according to a structure of an exhaust pipe of the internal combustion engine. The detected air-fuel ratio may deviate from an actual air fuel ratio sensor In a control unit described in Japanese Patent No. 2684011, since the deviation of detected air-fuel ratio due to the hit condition of the exhaust gas is not considered, the accuracy of the abnormality diagnosis based on the air-fuel ratio sensor output including the deviation may be deteriorated.